MARATTIACEAE 523 



see, the comparison is confirmed by reference to the petrified stems known 

 as Psaroniits. This consideration will justify our drawing together the 

 modern and the fossil forms into a comparison with a view to tracing 

 probable phyletic changes in the structure of the sorus, and a recognition 

 of an original type. 1 The definitely circumscribed sorus appears to be 

 a characteristic of the Marattiaceae, both ancient and modern. The form 

 of the sorus varied from circular to elongated, both in the fossils and in 

 living forms : there is no distinctive stratigraphical evidence to show which 

 type was the prior, but in the majority of the early fossils the sorus is 

 circular, with a small number of 

 sporangia. Further, the Pecop- 



terid is a relatively narrow-leaved CT'y .T ^ ^-^ 



type, while the leaves of Danaea 



and Kaulfussia are broad : if a 



widening of the leaf took place, 



followed . by extension of the A c 



Fie. 290. 



sorus, the result would be as in 



. . Danaeites saraepontanus, Stur. From the upper car- 



Danaea Or DanaeitCS : it abstriC- boniferous of the Saar district. A =0. fertile segment of 



. r .. , the last order. B = transverse section through two adjoin- 



tlOll OI the elongated SOri followed ing sori, or the hollow impression of them". C = below a 



, , , . sorus of sixteen sporangia ; above the impression of it. 



alSO, the result WOUld be as in (After Stur. From Engler and Vra.n\\, Nat. P/lanzenfaiii.) 



Kaulfussia. The evidence of the 



partial septa in Danaea, and the irregularity of size and segmentation of 

 the sporangia throughout the family, accords with the suggested extension 

 of an originally circular sorus with few loculi to produce the more or 

 less elongated sori of the living forms with more numerous loculi. 



A further point for discussion is the original relation of the sporangia 

 to one another in the sorus. Among both ancient and modern Marattiaceae 

 various gradations may be seen between such as have their sporangia 

 quite separate, and those in which they are synangially united. On this 

 point the palaeontological evidence would be consistent with either view, 

 for neither the synangial nor the polysporangiate state is distinctly the 

 prior in stratigraphical sequence. It becomes thus a question of comparison, 

 rather than of demonstration. As a matter of fact, all Marattiaceous sori 

 are synangia in the first phases of their ontogeny : many of them remain 

 so to maturity. It is only as the individual development proceeds that 

 the sporangia project as individual outgrowths in such a case as that of 

 Angiopteris. So far then as individual development bears on the question, 

 it would indicate the synangial state as the more primitive. Reasons have 

 already been shown for holding that a progressive septation accompanies 

 the extension of the sorus in the type of Danaea : a similar septation of 

 an enlarging initial spore-sac would produce the type of sorus seen, for 

 instance, in Ptychocarpus. Such an origin would consistently carry back to 

 an initial point that process of septation which is seen to be effective in 

 Danaea. From the synangial state thus produced the polysporangiate state 



: A more full statement of the arguments is given in Studies, iii., p. 67-77. 



